The present invention relates to a zoom lens system for finite distance photography, having a magnification of about 1/10 to 1/5 times. It relates, more particularly, to a zoom lens system in which the distance between the object surface and the image surface is maintained constant and finite, while enabling the power variation.
Conventionally well-known zoom lenses are classified into zoom lenses for still cameras having a magnification range from .infin.(magnification of 0) to about 1/10, and copying zoom lenses having a magnification range near the equal or unity magnification. The zoom lens system according to the present invention has an intermediate magnification range between the former and latter cases.
Comparing the zoom lens according to the present invention with the above described two known zoom lenses, the zoom lens of the present invention is similar to the still camera zoom lens in respect of the manner of moving the lens groups. Namely, the overall lens system is not moved as in the copying zoom lens but the respective lens groups are moved within the lens barrel. However, due to the fact that the distance between the object and the image is fixed, the present zoom system is similar to the copying zoom lens. Also, with respect to a lens performance, due to the fact that distortion is extremely small and a sufficient amount of marginal light is required, the zoom lens according to the present invention is similar to the copying zoom lens.
However, there is a significant difference between the present device and the above described lenses in that extremely high contrast and resolving power are required at marginal sections.
Thus, a comparison of the present zoom lens with zoom lenses which have different uses or object has been made. There are well known zoom systems having substantially the same magnification range as the present invention. Namely, there have been provided zoom systems in which a fixed focus lens is used, and in which a plurality of fixed focus lenses are rotated. The latter case is the so called "turret system". The former case has a disadvantage in that the distance between the object and image cannot be maintained constant and in addition, the amount of variation of the distance between the object and image is very large. The latter case has a disadvantage in that the magnification operation is not continuous but intermittent, and it is very difficult to adjust the system due to the manufacturing error of each lens element, i.e., to maintain the distance from the object to the image constant, and to adjust the system in order to obtain a predetermined magnification.